The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Internal combustion engines, especially automotive internal combustion engines, generally fall into one of two categories, spark ignition engines and compression ignition engines. Traditional spark ignition engines, such as gasoline engines, typically function by introducing a fuel/air mixture into the combustion cylinders, which is then compressed in the compression stroke and ignited by a spark plug. Traditional compression ignition engines, such as diesel engines, typically function by introducing or injecting pressurized fuel into a combustion cylinder near top dead center (TDC) of the compression stroke, which ignites upon injection. Combustion for both traditional gasoline engines and diesel engines involves premixed or diffusion flames that are controlled by fluid mechanics. Each type of engine has advantages and disadvantages. In general, gasoline engines produce fewer emissions but are less efficient, while, in general, diesel engines are more efficient but produce more emissions.
More recently, other types of combustion methodologies have been introduced for internal combustion engines. One of these combustion concepts is known in the art as the homogeneous charge compression ignition (HCCI). HCCI combustion, referred to hereinafter as controlled auto-ignition combustion mode, comprises a distributed, flameless, auto-ignition combustion process that is controlled by oxidation chemistry, rather than by fluid mechanics. In a typical engine operating in the controlled auto-ignition combustion mode, the intake charge is nearly homogeneous in composition, temperature, and residual level at intake valve closing time. Because controlled auto-ignition is a distributed kinetically-controlled combustion process, the engine operates at a very dilute fuel/air mixture (i.e., lean of a fuel/air stoichiometric point) and has a relatively low peak combustion temperature, thus forming extremely low NOx emissions. The fuel/air mixture for controlled auto-ignition is relatively homogeneous, as compared to the stratified fuel/air combustion mixtures used in diesel engines, and, therefore, the rich zones that form smoke and particulate emissions in diesel engines are substantially eliminated. Because of this very dilute fuel/air mixture, an engine operating in the controlled auto-ignition combustion mode can operate unthrottled to achieve diesel-like fuel economy.
At medium engine speed and load, a combination of valve profile and timing (e.g., exhaust recompression and exhaust re-breathing) and fueling strategy has been found to be effective in providing adequate heating to the cylinder charge so that auto-ignition during the compression stroke leads to stable combustion with low noise. One of the main issues in effectively operating an engine in the auto-ignition combustion mode has been to control the combustion process properly so that robust and stable combustion resulting in low emissions, optimal heat release rate, and low noise is achieved over a range of operating conditions. The benefits of auto-ignition combustion have been known for many years. The primary barrier to product implementation, however, has been the inability to control the auto-ignition combustion process.
A spark-ignition, direct-injection engine capable of operating in controlled auto-ignition combustion mode transitions between operating in an auto-ignited combustion mode at part-load and lower engine speed conditions and in a conventional spark-ignited combustion mode at high load and high speed conditions. There is a need to have a smooth transition between the two combustion modes during ongoing engine operation, in order to maintain a continuous engine output torque and prevent any engine misfires or partial-burns during the transitions These two combustion modes require different engine operation to maintain robust combustion. One aspect of engine operation includes control of the throttle valve. When the engine is operated in the auto-ignited combustion mode, the engine control comprises lean air/fuel ratio operation with the throttle wide open to minimize engine pumping losses. In contrast, when the engine is operated in the spark-ignition combustion mode, the engine control comprises stoichiometric air/fuel ratio operation, with the throttle valve controlled over a range of positions from 0% to 100% of the wide-open position to control intake airflow to achieve stoichiometry.
In engine operation, the engine air flow is controlled by selectively adjusting position of the throttle valve and adjusting opening and closing of intake valves and exhaust valves. Adjusting the opening, and subsequent closing, of intake and exhaust valves primarily takes the form of: phasing of opening (and subsequent closing) of the valve in relation to piston and crankshaft position; and, magnitude of the lift of the valve opening. On engine systems so equipped, opening and closing of the intake valves and exhaust valves is accomplished using a variable valve actuation (VVA) system that includes cam phasing and a selectable multi-step valve lift, e.g., multiple-step cam lobes which provide two or more valve lift profiles. In contrast to the continuously variable throttle position, the change in valve profile of the multi-step valve lift mechanism is a discrete change, and not continuous. When a transition between steps in the selectable multi-step valve lift is not effectively controlled, unwanted disturbances in engine air flow can occur, resulting in poor combustion, including misfire or partial-burns.
Therefore, there is a need to control the engine air flow during a transition between steps in the selectable multi-step valve lift, to achieve robust and stable combustion, low emissions, optimal combustion heat release rate, and low engine noise.